The invention relates generally to semiconductor processing equipment. More particularly, the invention relates to methods and apparatus for enabling liquid in an immersion lithography system to effectively be contained between a surface of a lens and a plane that is moved relative to the lens.
For precision instruments such as photolithography machines that are used in semiconductor processing, factors that affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument such as an immersion lithography exposure system is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly.
In an immersion lithography system, a liquid is provided between a lens and the surface of a wafer in order to improve the imaging performance of the lens. The use of liquid allows a numerical aperture associated with the lens, i.e., an effective numerical aperture of the lens, to essentially be increased substantially without altering characteristics of the lens, since a liquid such as water generally has a refractive index that is greater than one. In general, a higher numerical aperture enables a sharper image to be formed on the wafer. As will be appreciated by those skilled in the art, a high refractive index liquid allows for a high numerical aperture of the lens because an effective numerical aperture of a lens system of an immersion lithography system is generally defined to be approximately equal to the sine of an angle of diffraction of light that passes through a lens and reflects off a surface multiplied by the refractive index of the liquid. Because the refractive index of the liquid is greater than one, the use of liquid allows the effective numerical aperture of the lens to be increased, thereby enabling the resolution associated with the lens to essentially be improved.
Within most conventional lithography systems, air is present between a lens and a surface that passes under the lens, e.g., the surface of a wafer. In such systems, the numerical aperture associated with the lens is often in the range of approximately 0.8 to 0.9. Increasing the numerical aperture of a lens to achieve an improved resolution is generally impractical, because the diameter of a lens generally must be increased, which adds significant difficulty to a lens manufacturing process. In addition, the numerical aperture of a lens in air is theoretically limited to one, and, in practice, is limited to being somewhat less than one. Hence, immersion lithography systems enable the effective numerical aperture of a lens to be increased substantially beyond what is possible with a lens in air.
FIG. 1 is a diagrammatic cross-sectional representation of a portion of an immersion lithography apparatus. An immersion lithography apparatus 100 includes a lens assembly 104 that is positioned over a wafer table 112 that supports a wafer 108. Wafer table 112 is arranged to be scanned or otherwise moved under lens assembly 104. A liquid 116, which may be water in a typical application that uses approximately 193 nanometers (nm) of radiation, is present in a gap between lens assembly 104 and wafer 108. In order to effectively prevent liquid 116 from leaking out from under lens assembly 104, i.e., to effectively laterally contain liquid 116 between lens assembly 104 and wafer 108, a retaining ring 120 may be positioned such that retaining ring 120 enables liquid 116 to remain between lens assembly 104 and wafer 108, and within an area defined by retaining ring 120.
While retaining ring 120 is generally effective in containing liquid 116 when lens assembly 104 is positioned such that a small gap between retaining ring 120 and a surface of wafer 108 is maintained, for a situation in which at least a part of retaining ring 120 is above wafer 108, liquid 116 may leak out from between lens assembly 104 and wafer 108. By way of example, when an edge of wafer 108 is to be patterned, lens assembly 104 may be substantially centered over the edge such that a portion of retaining ring 120 fails to maintain the small gap under the bottom surface of retaining ring 120, and liquid 116 is allowed to leak out from between lens assembly 104 and wafer 108. As shown in FIG. 2, when lens assembly 104 is positioned such that at least part of a bottom surface of retaining ring 120 is not in contact with wafer 108, liquid 116 may not be contained in an area defined by retaining ring 120 between lens assembly 104 and wafer 108.
In an immersion lithography apparatus, a wafer table may support sensors and other components, e.g., a reference flat that is used to calibrate automatic focusing operations. Such sensors and other components generally may be positioned beneath a lens at some point. That is, sensors and other components associated with a wafer table may be occasionally positioned beneath a lens during the course of operating the lens and the wafer table. While the use of a retaining ring may prevent liquid from leaking out of a gap between a lens assembly and the top surface of the wafer, liquid may leak out from between the lens assembly and top surfaces of sensors and other components when the lens assembly is positioned over the sensors or other components.
FIG. 3 is a block diagram representation of a wafer table that supports a sensor and a wafer holder that holds a wafer. A wafer table 312 supports a wafer holder 310 that is arranged to hold a wafer (not shown), a sensor 350, and an interferometer mirror 352. Sensor 350 may be used through a lens (not shown) with liquid (not shown) between the lens and sensor 350. However, liquid will often flow out of the gap between a lens (not shown) and sensor 350 particularly when an edge of sensor 350 is positioned substantially beneath a center of the lens. The effectiveness of sensor 350 may be compromised when sensor 350 is designed and calibrated to operate in a liquid, and there is insufficient liquid present between a lens (not shown) and sensor 350. Further, when liquid (not shown) flows out of the gap between a lens (not shown) and sensor 350, the liquid that flowed out of the gap is effectively lost such that when the lens is subsequently positioned over a wafer (not shown) supported by wafer holder 310, the amount of liquid between the lens and the wafer may not be sufficient to enable the effective numerical aperture of the lens to be as high as desired. Hence, when liquid is not successfully contained between a lens (not shown) and sensor 350 while sensor 350 is at least partially positioned under the lens, an overall lithography process that involves the lens and sensor 350 may be compromised.
Therefore, what is needed is a method and an apparatus for allowing liquid to be maintained in a relatively small gap defined between a surface of a lens and a surface of substantially any sensors or components that are supported by a wafer table. That is, what is desired is a system that is suitable for preventing liquid positioned between a lens and substantially any surface on a wafer table that is moved under the lens from leaking out from between the lens and the surface.